METHOD OF MANUFACTURING THIN CAST PIECE THROUGH CONTINUOUS CASTING

Abstract: Semi-solidified metallic slurry is supplied from a semi-solidified metallic slurry manufacturing apparatus to a double-roll type strip continuous caster through a discharge nozzle, which is provided with heating means for heating the nozzle itself, to be quenched and solidified so that a thin cast piece of a good quality is obtained through continuous casting which micronizes a structure and precipitate dispersion.

Description: TECHNICAL FIELD This invention relates to a method of producing thin cast sheets (band-shaped) through continuous casting of a semi-solidified metal (alloy) slurry as a raw material for high-quality and low-cost sheets in which the formation of fine grain structure and the fine dispersion are conducted to mitigate segregation and surface cracking and improve the workability (see e.g. :JP-A-01027751). BACKGROUND ART A generally satisfactory technique for continuously casting semi-solidified metal slurry has not hitherto been established. The main reason is due to the fact that the semi-solidified metal slurry is solidified by slight loss of heat. That is, the temperature of the semi-solidified metal slurry in the production step of the semi-solidified metal slurry is naturally lower than the liquidus line for the metal, so that when heat is removed from the semi-solidified metal slurry on having the slurry contact an inner wall surface of a discharging device (e.g. discharge nozzle) during the discharge of the semi-solidified metal slurry, even if the heat removal is slight, adhesion of high melting point component (e.g. Al 2 O 3 or the like) in the semi-solidified metal slurry to the wall surface of the device is caused with, solidification adhesion of the semi-solidified metal slurry itself on the wall surface taking place. Such solidification shell adheres to the wall surface. In the discharging device for the semi-solidified metal slurry, therefore, there is a basic problem that is apt to cause trouble, such as impossibility of controlling discharge amount, clogging of nozzle and the like based on the adhesion of the solidification shell. To this end, it is important to solve this problem in order to conduct the continuous casting by supplying semi-solidified metal slurry to a continuous casting machine through a discharge nozzle. In general, an immersion nozzle is used in the supply of molten metal from a tundish into a continuously casting mold. As regards the immersion nozzle, in order to avoid precipitation adhesion of the high melting point component to the inner wall surface of the nozzle during the supply of molten metal or solidification adhesion of molten metal itself to the wall surface resulting from the taking-off of heat through the nozzle, or so-called clogging trouble of the nozzle runner due to the adhesion of solidification shell, there are known, for example, a countermeasure wherein an electric heating body is inserted into the nozzle to preheat the nozzle from its inner side, as disclosed in JP-A-63-286268 (method of heating tundish nozzle), a countermeasure providing for preheating the nozzle from its inner side by means of a burner, a countermeasure wherein the nozzle body is made from an electrically conductive refractory material and a current is directly supplied to heat the nozzle as disclosed in JP-B-63-24788, a countermeasure wherein an induction heating coil is arranged around the outer periphery of the nozzle to heat molten metal passing through the nozzle by induction heating, and others. However, it has been confirmed that all of the above countermeasures are unsuitable when the above immersion nozzle for molten metal is used as a discharge nozzle for the semi-solidified metal. Thus, when preheating with the electric heating body or the burner, it is in practice, difficult to raise the preheating temperature up to a temperature exceeding 1000°C at the inner wall surface of the nozzle and also there is, particularly, a problem that the heating cannot be conducted during the passing of the semi-solidified metal through the nozzle. In the actual operation, therefore, the temperature of the inner wall surface of the nozzle specially preheated drops in the discharge of the semi-solidified metal before the completion of the discharge and a part of the semi-solidified metal passing through the nozzle becomes a solidification shell adhered to the wall surface by removal of heat through the wall surface to thereby cause the clogging of the nozzle. In case of directly supplying the current to heat the nozzle, there is the problems of risk of electric leakage being high due to the embedding of the special electrode in the nozzle and the satisfactory heating temperature cannot be obtained only by heat conduction of the nozzle itself because the electrode cannot be embedded in the top portion of the nozzle to be immersed. Since the induction heating system has an induction heating coil arranged around the outer periphery of the nozzle mainly to reheat molten metal passing through the nozzle, the applied frequency presently adopted is less than 10 kHz, so that the induction current is absorbed by the semi-solidified metal having a conductivity higher than that of the nozzle body during the passing of the semi-solidified metal through the nozzle and hence it has been confirmed that the desired effect of heating the nozzle body to prevent the removal of heat from the semi-solidified metal can not be achieved. Furthermore, the reheating of the semi-solidified metal reduces the solid fraction and results in fine primary solid particles being incorporate, thereby degrading quality. This system is also unsuitable from this point. On the other hand, metal (alloy) materials used as a raw material for ingots or slabs cast from their melts may have unavoidable problems for production, quality and economical reasons. Such materials are exemplified by austenitic stainless steel, boron-containing austenitic stainless steel, ferritic stainless steel, martensitic stainless steel, silicon steel for electromagnetic sheets, phosphor bronze alloy and high-Sn copper alloy for superconducting material. These metal materials have the following problems. In general, austenitic stainless steel has a large susceptibility to cracking during hot working as compared with the ferritic stainless steel. Therefore, a thin sheet of this steel is commonly produced by blooming a steel ingot into a slab, removing cracks generated on the surface of the slab by grinding work, and then subjecting the slab to hot rolling and cold rolling. However, the removal of the cracks from the slab surface disadvantageously and largely decreases the product yield. In order to solve the above problem, it is attempts have been made to directly produce a cast sheet corresponding to the slab by continuous casting. Even when this process is applied, it is difficult to avoid the surface cracking, so that the work of removing the cracks by grinding is not yet avoided. Boron-containing austenitic stainless steel is characterized by having a large thermal neutron absorbability of B included and has excellent corrosion resistance, so that it is favorable as a thermal neutron shielding material. However, B in the steel reacts with Fe or Cr to form a boride as an intermetallic compound and the resulting boride considerably degrades the hot workability, so that there is the problem that it is very difficult to produce steel sheets by hot rolling as the B content increases. It is desired to solve this problem. As a technique for solving the above problem, there has been proposed a countermeasure for improving hot workability by adjusting a ratio of Al and N contents in boron-containing austenitic stainless steel, for example, in JP-A-57-45464 (boron-containing austenite stainless steel for atomic reactor having excellent hot workability). Furthermore, there has been proposed a countermeasure for improving the hot workability by adding V to the boron-containing stainless steel in JP-A-55-89459 (boron-containing stainless steel having excellent corrosion resistance and workability). However, little improvement in hot workability by the addition of the alloying component is to be expected and it is difficult to produce boron-containing austenitic stainless steel sheet by the usual hot rolling. Moreover, JP-A-4-236716 (method of producing hot rolled sheets of B-containing austenitic stainless steel) proposes a hot rolling method wherein the temperature and draught in the blooming are specified to define the heating temperature of the resulting bloomed slab, rolling end temperature and final finish rolling rate as a method of preventing hot rolled cracks in the boron-containing austenitic stainless steel. In this method, however, coarse boride is produced in the solidification of steel ingot, so that the effect of preventing the hot rolled cracks can be expected only in a low B-containing steel in which the B content is not more than 1.2 wt.%, which is not sufficient. In ferritic stainless steel, columnar crystal is apt to be grown at the solidification step. When a cast slab having such columnar crystal grown therein is used as a raw material and rolled and then subjected to a forming work with a press or the like, uneven defects called ridging occur on the surface of the resulting thin steel sheet. The occurrence of the ridging results from the fact that the columnar crystal grown from the surface of the cast sheet toward its central portion forms a texture having a strong orientation when rolling and cold rolling take place. As a method of preventing the occurrence of the ridging, a method is used wherein the solidification structure is improved by decreasing the casting temperature during continuous casting or electromagnetic agitation of molten metal, or by controlling the hot rolling conditions or heat treating conditions. With a cast sheet of about 200 mm thickness obtained by the continuous casting, however, it is difficult to sufficiently prevent the growth of columnar crystal even by the adjustment of the casting temperature or the use of the electromagnetic agitation of molten metal because the solidification rate is slow. Furthermore, the occurrence of ridging cannot be prevented by the subsequent control of the rolling conditions or the heat treating conditions. On the other hand, a method of preventing the occurrence of the ridging by reducing the thickness of the cast sheet. For instance, JP-A-62-54017 (method of producing thin cast sheets of Cr-based stainless steel) proposes a method wherein the Cr-based stainless steel is cast into a thin sheet and then subjected to a given cooling, working and heat treatment to prevent the occurrence of ridging. Furthermore, JP-A-62-176649 (method of producing thin sheet strip of no-roping ferritic stainless steel) proposes a method wherein a thin sheet strip of not more than 5 mm thickness is cast from molten metal by single roll or twin roll process and then subjected to annealing, cold rolling and annealing to prevent the occurrence of roping (ridging). However, the method of thinning the thickness of the cast sheet to control the occurrence of the ridging is effective in that the solidification rate is increased, but can not completely control the growth of columnar crystal because molten metal supplied has a superheat (temperature difference between molten metal temperature and liquidus line). Moreover, the reduction ratio is decreased and the breakdown of the solidification structure is insufficient, so that special cooling conditions, rolling conditions and heat treating conditions are required. With martensitic stainless steel, particularly high carbon Cr martensite, primary carbide is precipitated in net form owing to the hyper-eutectoid steel. With a conventional continuous cast slab, coarse carbide is inhomogeneously precipitated, accompanied with macrosegregation to degrade the product quality. To this end, a countermeasure to the formation of fine carbide at the working stage has been considered, but is not yet sufficient. As to the silicon steel, there is used a so-called molten metal process wherein molten metal is cast in an ingot-forming mold or a continuous casting mold. Recently, there has been developed a so-called rapid quenching process wherein a very thin silicon steel sheet (thin ribbon) is produced by rendering the steel sheet amorphous by means of a water-cooled single roll. When the cast ingot or cast slab is produced by the above molten metal process, columnar crystal grows from the surface of the cast body towards its center to form a giant crystal as a solidification structure. As a result, component segregation is caused in the central portion of the cast body by the growth of the columnar crystal. With grain oriented silicon steel, there is adopted a method of precipitating fine MnS, MnSe and the like as an inhibitor on movement of the crystal grain boundary in the secondary recrystallization to improve the selectivity of crystal growing orientation. However, if the aforementioned component segregation is caused, the amount of precipitated MnS, MnSe and the like becomes large in the segregated portion, which undesirably lowers the effect as the inhibitor. As a countermeasure, MnS, MnSe and the like are again dissolved so as to finely reprecipitate before the hot rolling, with the soaking temperature in the solution treatment being about 1400°C, considerably higher than that of the other steels. Therefore, many problems are caused such as accumulation of oxide scale in the heating furnace, loss of the furnace structure and increase of energy loss. In the rapid quenching method, it is required to include a large amount of a specific component (B or the like) for the formation of amorphous state and there are still problems in the mass production and stable operation, so that only a part of product is actually industrialized. On the other hand, the larger the Si content, the better the magnetic properties such as maximum permeability and the like. The properties are at a maximum at 6.5 wt%. However, the elongation rapidly decreases when the Si content is not less than 2.5 wt% and is substantially zero at 5 wt%. Thus, as the Si content increases, brittleness becomes rapidly remarkable, so that it is difficult to conduct the cold rolling of silicon steel containing Si of not less than about 3.5 wt%, and also the hot rolling is impossible at such content exceeding 5 wt%. Therefore, the Si content of mass-produced silicon steel is restricted to not more than 3.5 wt% except for only some silicon steel produced by a special process. In general, a phosphor bronze alloy is apt to cause segregation at the solidification stage, and an Sn rich layer called "tin sweat" is apt to be formed on a surface of a cast ingot. And also, a δ-phase, Cu-Sn intermetallic compound, is formed in this rich layer, which results in cracking during the subsequent work. To this end, the phosphor bronze alloy plate of not less than 15 mm thickness is usually produced by continuous casting, and thereafter subjected to surface grinding by about 2.5 mm at every side of the plate to remove tin sweat and then introduced into the soaking treatment → cold rolling steps. In the conventional method of subjecting continuously cast phosphor bronze alloy plate to grinding treatment, however, the grinding margin is 5 mm in total of front and back surfaces, so that the product yield is largely reduced and throughput is obstructed in view of this operational step. In order to avoid "tin sweat" as a problem in the production of this type of the sheet, it is known that it is effective to control the solidification segregation by rapidly quenching molten metal in the continuous casting. Even if the solidification rate is made large, the structure forms a dendritic columnar crystal structure extending from the surface layer of the cast sheet towards its central portion, so that the occurrence of tin sweat and the formation of δ-phase are inevitable at this region and hence the grinding treatment for removing this portion could not yet be omitted. High-Sn copper alloy containing Sn at not less than 8 wt% is used in the production of Nb 3 Sn superconducting material. In the production of very fine multicore Nb 3 Sn superconducting wire, it is said that the Sn content in Cu-Sn alloy used as a matrix alloy is preferabley large in order to shorten the diffusion distance and improve performance. However, when the Sn content is not less than 8 wt% the segregation is conspicuous and the grain boundary becomes brittle due to the precipitation of δ-phase, which renders hot rolling and cold rolling impossible. DISCLOSURE OF THE INVENTION It is an object of the invention to provide a casting method for thin-cast sheets made from continuously supplied molten metal and particularly a method of producing such thin cast sheets having high quality and low cost. According to this invention, there is provided a method of producing thin cast sheets by continuous casting in a continuous production device having an upper portion and a lower portion, which comprises continuously feeding molten metal into a said upper portion of the continuous production device, agitating said molten metal in said continuous production device, cooling said molten metal during said agitating to form in said continuous production device a semi-solidified metal slurry and feeding the semi-solidified metal slurry through a discharge nozzle arranged at the bottom portion of the continuous production device, said nozzle being heated, onto a twin toll type continuous strip caster at which the slurry is rapidly quenched and cast to form a thin cast metal sheet having a fine structure and a dispersed precipitate therein, in which method said cooling is carried out to yield a semi-solidified metal slurry having a solid fraction of 0.01 to 0.40 in which fine non-dendritic primary solid particles are suspended and in that said nozzle is heated through high frequency induction heating utilizing a frequency of 40-200 kHz so that not less than 80% of the supplied induction, current is applied to said nozzle. In such method, preferably use is made of an electromagnetic agitating system. Alternatively, use may be made of an agitator rotating system. When carrying out the method of the invention , the discharge nozzle is preferably made from alumina graphite having a specific resistance of 5000 µΩ·cm - 12000 µΩ·cm. Preferably the nozzle has a wall thickness of 15-40mm. Casting is preferably carried out so that the thin cast metal sheet has a thickness of not more than 10mm. Preferred examples of molten metals to which the invention may be applied are: a) austenitic stainless steel, the casting being carried out so that the thin metal sheet exhibits a fine dispersion of P and S. b) a boron-containing austenitic stainless steel containing B:0.5-4.0 wt.%, P and S so that the thin metal sheet exhibits a fine dispersion of P, S in the form of boride c) a ferritic stainless steel so that the formation of columnar crystal is prevented d) martensitic stainless steel so that a fine dispersion of carbide is obtained e) a silicon steel containing Si: 3.0-6.5 wt.% and Mn: to not more than 2.5 wt.% so that the thin metal sheet exhibits a structure and a fine dispersion of a Mn-containing compounds, f) a phosphor bronze alloy containing Sn: 3.5-9.0 wt.% and P: 0.03-0.35 wt.% so that formation of columnar crystal and fine structure in said thin metal sheet is prevented g) a high Sn copper alloy containing Sn: 8-20 wt.% so that the formation of columnar crystal and fine structure in said thin metal sheet is prevented. The function and effect of the invention will be described below. According to the invention, molten metal is agitated under cooling to continuously produce a semi-solidified metal slurry of a solid-liquid mixed phase with fine non-dendritic primary solid particles suspened therein, which is fed through the discharge nozzle provided with the means for heating the nozzle itself onto the twin roll type continuous strip caster to conduct the rapid quenching and continuous casting for producing a fine structure and with the precipitate dispersed therein, whereby a thin cast sheet having a good quality is produced. Therefore, the slurry can continuously be fed into the twin roll type continuous strip caster by using the discharge nozzle provided with the means for heating the nozzle itself without causing troubles such as nozzle clogging due to the adhesion of solidification shell and the like and degrading the quality due to the overheating of the semi-solidified metal slurry passing through the nozzle, whereby the thin cast sheet can be produced by the continuous casting without difficulty. The production of the above semi-solidified metal slurry and the continuous casting thereof will now be described in detail. As the agitation system in the production of the semi-solidified metal slurry, an electromagnetic agitating system is favorable from the viewpoint that it is applicable to high melting point metal, it been possible for the semi-solidified metal slurry be produced up to a solid fraction of 0.4, maintenance being relatively simple. Alternatively, an agitator rotating system can be used in which the agitator is mechanically rotated. These systems facilitate the continuous production of the semi-solidified metal slurry. However, it is impossible to continuously feed the semi-solidified metal slurry produced by these systems into the twin roll type continuous strip caster through a conventional nozzle without clogging the nozzle. The inventors have made various experiments and studies and found that it is effective and essential to positively and steadily heat the nozzle as the discharge nozzle in order to solve problems such as nozzle clogging and the like. For this purpose, the heating is carried out by high frequency induction heating at a frequency of 40 kHz - 200 kHz. For this purpose, a high frequency induction heating coil will generally be disposed around the outer periphery of the discharge nozzle. The nozzle itself may be made from alumina graphite having a specific resistance of 500 µΩ·cm - 12000 µΩ·cm. In this way it is possible for the nozzle, itself, to be heated to a temperature of 1500°C and also for the semi-solidified metal slurry to be made from a high melting point metal which can continuously be fed into the twin roll type continuous strip caster without problems such as nozzle clogging and the like while retaining heat received by thermal conduction through the nozzle wall. In the heating of the discharge nozzle by the high frequency induction heating coil, the frequency is selected within a range of 40 kHz - 200 kHz. By adopting such a frequency a surface skin effect is generated, whereby a greater part of induction current can be concentrated in the nozzle body. In practice, when the frequency is selected so as to apply not less than 80% of the induction current to the nozzle body, inconveniences such as the decrease of solid fraction produced by reheating of the semi-solidified metal, accumulation of fine primary solid particles and the like are practically resolved without raising the temperature of the semi-solidified metal passing through the nozzle by the induction heating, whereby degradation of the quality of the discharged semi-solidified metal can be prevented. When the discharge nozzle of a ceramic refractory material having a high conductivity is heated by the high frequency induction heating, a relation between depth to which 80% of the induction current flows or penetration depth (t) at which 80% of induction current flows and frequency applied (f) exists which is represented by the following equation (1): f = K x R/t 2 where f: frequency (kHz) K: proportional constant (kHz/Ω) R: specific resistance of nozzle (Ω·mm) t: penetration depth (mm). When the frequency is evaluated by substituting practical properties of the discharge nozzle in the equation (1), a graph shown in Fig. 1 is obtained, from which the frequency is obtained within a range of 40 kHz - 200 kHz because the wall thickness of the usual discharge nozzle is 15-40 mm. Moreover, Fig. 1 is a graph showing the relation between the penetration depth through which 80% of induction current flows and the frequency in the high frequency induction heating of the discharge nozzle. Therefore, the frequency applied in the high frequency induction heating is 40 kHz - 200 kHz. As material of the nozzle in the high frequency induction heating, alumina graphite as a high electrically conductive refractory substance is suitable because it possesses resistance to fusion loss and thermal shock resistance. In the alumina graphite, the electrical conductivity can be increased by increasing the amount of graphite. From the point of view of resistance to fusion loss, thermal shock resistance, oxidation resistance, hot deflective strength and the like, the graphite amount is suitably within a range of 10% - 30%, which corresponds to a specific resistance of 500 µΩ·cm - 12000 µΩ·cm. Concerning the amount of solid phase in the semi-solidified metal slurry fed through the twin roll type continuous strip caster, this affects the formation of fine structure and the dispersion of fine precipitates. When the solid fraction is less than 0.01, a coarse columnar crystal structure may partially be produced. Hence, there is a lower limit of 0.01. On the other hand, when the solid fraction exceeds 0.40, the viscosity of the slurry rises sharply and handling is difficult, so that the upper limit is 0.40 for reasons of operability. As regards the thickness of the thin cast sheet, when the thickness exceeds 10 mm, the solidification rate is slow, so that the formation of fine structure and the dispersion of fine precipitates may not be sufficient and hence there are caused the following problems in, for example, each of the metal materials which has been considered. In austenitic stainless steel and boron-containing austenitic stainless steel, when granular crystal produced in the solidification is coarsened to form a liquid film at the crystal grain boundary, which is apt to cause the surface cracking and the ridging of the product. In ferritic stainless steel, granular crystal produced in the solidification is apt to be coarsened into columnar crystal, and the risk of generating ridging in the product becomes large. In martensitic stainless steel, coarse carbide is apt to be produced in the central portion of the thin cast sheet and degrades the properties. In silicon steel for electromagnetic steel sheets, the formation of fine structure and the dispersion of fine MnS, MnSe and the like are apt to be insufficient. That is, the occurrence of the columnar crystal brings about the occurrence of the ridging in the product and the increase of scattering of crystal orientation texture, and also the effect of decreasing the temperature of solution treatment before the hot rolling in the grain oriented silicon steel sheet and the effect of improving the workability of high Si steel can not be expected. In phosphor bronze alloys and high Sn copper alloys, the effect of preventing the occurrence of so-called tin sweat is less and Sn, P rich segregation is apt to be generated in the central portion of the thin cast sheet in the thickness direction to form δ-phase resulting in the work cracking. Therefore, the thickness of the thin cast sheet is desirably to be not more than 10 mm. Next, the behaviour in the production of thin cast sheets of each of the metal materials subjected to continuous casting will be described. 1 ○ Production of thin cast sheets of austenitic stainless steel Cracking of austenitic stainless steel during hot working results from the fact that the segregation of P and S as impurity in steel is fairly large as compared with the case of ferritic stainless steel. That is, a P, S rich layer is apt to be formed on a front surface of the solidification shell formed in the solidification step of the austenitic stainless steel and this layer finally produces a rich segregation at the crystal grain boundary while raising the concentration of P, S with the advance of the solidification. The rich segregation portion is low at the solidification point and partly fuses when being reheated to a hot working temperature. This is the starting point for breakage in hot working. Since the hot cracking inherent to the austenitic stainless steel results from so-called liquid film brittleness, it is caused even by deformations at bulges, bend portions and relaxation portions of the thin cast sheet in the continuous casting in addition to the hot working. On the basis of the above causes of cracking, it is considered that in the continuous casting of the austenitic stainless steel, the segregation can be mitigated to control the occurrence of cracking by rapidly quenching molten steel. In fact, improvement to a certain extent is attained by rapid quenching, but the surface cracking cannot be avoided only by simply increasing the solidification rate of molten steel. This is due to the fact that the solidification structure of the thin cast sheet is a coarse columnar crystal structure extending from the surface layer of the thin cast sheet toward the center thereof in the thickness direction and the liquid film produced in the crystal grain boundary is largely mitigated as a whole but a large liquid film is locally formed. According to the invention, the semi-solidified metal slurry obtained by agitating in a temperature region of not higher than liquidus line but not lower than solidus line is cast by using the twin roll type continuous strip caster, so that the formation of coarse columnar crystal structure is controlled and a mixed structure consisting of fine granular solid particles (hereinafter referred to as primary solid particles) and granular crystal can be obtained, whereby the surface cracking inherent to the austenitic stainless steel is advantageously avoided. Also, the continuous casting of the semi-solidified metal slurry of austenitic stainless steel with the twin roll type continuous strip caster has the following advantages as compared with when otherwise using molten metal in addition to the avoidance of surface cracking in the thin cast sheet. (1) The productivity can be increased because the solidification rate is large. (2) Since a part of latent heat in the solidification is released, the thermal loading of the cooling roll is mitigated and it is possible to prolong the service life of the roll. (3) Since the slurry has an adequate viscosity, the surface properties can be improved. In addition to the above (1)-(3), since the primary solid particles are suspended in a liquid phase of the semi-solidified metal slurry, the cast structure is a mixed structure consisting of the primary solid particles existing as a solid phase in the slurry and fine granular crystal produced in the casting and hence there is no coarse columnar crystal structure formation as in the continuous casting of molten metal. Moreover, these advantages are obtained even with the following metal materials. 2 ○ Production of thin cast sheets of boron-containing austenitic stainless steel There have been made various studies and examinations on the hot workability of the boron-containing austenitic stainless steel and it has been found that the degradation of the hot workability results from the fact that borides being mainly compounds of B with Fe and Cr produced at the austenite crystal grain boundary at the solidification stage are starting points for breaking the austenite crystal grain boundary in the hot working and further the boride produced in the crystal grain boundary becomes much and coarse as the crystal grain size of austenite becomes large and degrades the hot workability. Therefore, a method wherein the austenite crystal grains produced at the solidification stage is made fine as finely disperse boride precipitates at the grain boundary is effective to improve the hot workability. In the usual casting method for steel ingots and the continuous casting method for the thin cast sheets having a thickness of about 150 mm, the solidification rate is slow, so that it is difficult to obtain fine austenite crystal grains at the solidification stage. Further, one may consider method wherein the thin cast sheet is directly produced from molten metal with there being rapid quenching and omission of hot rolling. In this method, the solidification rate increases, but coarse columnar crystal is formed from the surface of the thin cast sheet toward the center thereof because the supplied molten metal is superheated (temperature difference between molten metal temperature and liquidus line), so that the formation of fine austenite crystal grains cannot be attained. Moreover, since coarse boride is formed at the grain boundary of columnar austenite crystal grains, cracking is frequently generated in the bend-unbend portions during the casting of the thin cast sheet or in the coiling portion, so that the sheet is difficult to use as a raw rolling material for the production of steel sheets. Under the above circumstances, a semi-solidified metal slurry of austenitic stainless steel having a B content of 0.5-4.0 wt% obtained by agitating at a solid-liquid coexisting region is rapidly quenched and cast in the twin roll type continuous strip caster, so that the formation of columnar crystal can completely be prevented in the resulting thin cast sheet and the structure is a mixed structure consisting of fine and columnar primary solid particles suspended in the semi-solidified metal slurry and the fine granular crystal produced by rapidly quenching the liquid phase of the semi-solidified metal slurry with the surface of the roll and hence the boride precipitated in the austenite crystal grain boundary can be finely dispersed. As a result, hot workability is excellent and cracking can be prevented at the casting step for the thin cast sheet and raw rolling material for cold rolled steel sheets, omitting hot rolling, can be obtained. Next, the invention will be described with the following experiment. Each of molten metal (superheat ΔT: 60°C) and semi-solidified metal slurry (solid fraction: 0.2) of SUS304 austenitic stainless steel containing B: 2.0 wt% is supplied to a twin roll type continuous strip caster to form a thin cast sheet having a thickness: 8 mm, and then the metal structure of a section of the resulting thin cast sheet is examined. Microphotographs of these metal structures are shown in Fig.2 and Fig. 3, respectively. Fig. 2 is a microphotograph of the metal structure of the thin cast sheet made from molten metal of SUS304 austenitic stainless steel containing B: 2.0 wt%, and Fig. 3 is a microphotograph of the metal structure of the thin cast sheet made from semi-solidified metal slurry of SUS304 austenitic stainless steel containing B: 2.0 wt%. As seen from these figures, the metal structure at a section of the thin cast sheet made from molten metal (Fig. 2) is comprised of coarse columnar crystal produced from the surface of the thin cast sheet toward the center thereof, while the metal structure at a section of the thin cast sheet made from the semi-solidified metal slurry (Fig. 3) is comprised of fine austenite crystal grains. Further, when thin cast sheets having a thickness: 8 mm are cast in a twin roll type continuous strip caster with varyiation of superheat of molten metal and a solid fraction of semi-solidified metal slurry in SUS304 austenitic stainless steel containing B: 2.1 wt%, respectively, and then the area ratio of columnar crystal occupying the section of the thin cast sheet is measured with respect to the resulting thin cast sheets, the measured results shown in Fig. 4 are obtained. Fig. 4 is a graph showing the relation between the superheating of molten metal and the ratio of solid fraction of semi-solidified metal slurry to the occupying area of columnar crystal in a section of the thin cast sheet in SUS304 austenitic stainless steel containing B: 2.1%. As seen from this figure, when the superheat ΔT of the supplied molten metal is 0°C (solid fraction: 0), columnar crystal is somewhat formed, while when the solid fraction of the semi-solidified metal slurry is not less than 0.1, columnar crystal is not formed, which shows that the formation of columnar crystal can completely be controlled by the method according to the invention. As to the chemical composition, B is added to austenitic stainless steels having excellent corrosion resistance and heat resistance. The B content is required to be not less than 0.5 wt% in order to effectively develop neutron shielding effect, while when it exceeds 4.0 wt%, it is difficult to completely control the occurrence of the cracking in the casting even by using the method according to the invention. Therefore, the B content is within a range of 0.5-4.0 wt%. Moreover, the invention is preferably applied to B-added SUS304, SUS304L, SUS309S, SUS310S and the like but is advantageously applicable to steels obtained by adding B to the other austenitic stainless steels. 3 ○ Production of thin cast sheets of ferritic stainless steel Ridging generated in ferritic stainless steel sheet results from the fact that the solidification structure of the thin cast sheet as a raw rolling material forms a coarse columnar crystal. According to the invention, the semi-solidified metal slurry obtained by agitating at a temperature region of not higher than liquidus line but not lower than solidus line is rendered into a thin cast sheet by rapid quenching in the twin roll type continuous strip caster to prevent the formation of columnar crystal, so that the resulting thin cast sheet may have no formation of columnar crystal and the structure thereof is a mixed structure consisting of fine and columnar primary solid particles suspended in the semi-solidified metal slurry and the fine granular crystal produced by rapidly quenching the liquid phase of the semi-solidified metal slurry with the surface of the roll. Therefore, there is no ridging in the forming work of the steel sheet made from the thin cast sheet. Next, the invention will be described with the following experiment. Thin cast sheets having a thickness: 6 mm are cast in the twin roll type continuous strip caster by varying superheat of molten metal and solid fraction of semi-solidified metal slurry in SUS430 ferritic stainless steel, and then the area ratio of columnar crystal occupied in the section of the thin cast sheet is measured with respect to the resulting thin cast sheets. The measured results are shown in Fig. 5. Fig. 5 is a graph showing a relation of the superheat of molten metal and the solid fraction of semi-solidified metal slurry to the occupying area ratio of columnar crystal in a section of the thin cast sheet of SUS430 ferritic stainless steel. As seen from this figure, when the superheat ΔT of the supplied molten metal is 0°C (solid fraction: 0), columnar crystal is somewhat formed, while when the solid fraction of the semi-solidified metal slurry is not less than 0.1, columnar crystal is not formed, which shows that the formation of columnar crystal can completely be controlled by the method according to the invention. 4 ○ Production of thin cast sheets of martensitic stainless steel Martensitic stainless steel, particularly high-carbon Cr martensitic steel is a hyper-eutectoid steel and primary carbide is precipitated therein. A greater amount of coarse carbide is precipitated in a central portion of the slab creating macrosegregation to degrade the properties. According to the invention, the semi-solidified metal slurry obtained by agitating at a temperature region of not higher than the liquidus line but not lower than solidus line is rendered into a thin cast sheet by rapidly quenching in the twin roll type continuous strip caster, so that the thin cast sheet is a mixed structure consisting of fine and columnar primary solid particles suspended in semi-solidified metal slurry and fine granular crystal produced by rapidly quenching the liquid phase of the semi-solidified metal slurry with the surface of the roll, and the macrosegregation becomes less. Therefore, there are obtained thin cast sheets having less coarse carbide and less surface cracking. 5 ○ Production of thin cast sheets of silicon steel When the silicon steel for electromagnetic steel sheets is rendered into a thin cast sheet by casting the semi-solidified metal slurry obtained under agitation at the solid-liquidus coexisting region with the use of the twin roll type continuous strip caster, the solidification structure is a uniform granular structure for the whole of the thin cast sheet, and the solidification time is very short as compared with the casting of molten metal, so that the crystal grain is small and the component segregation is largely mitigated and the precipitates of MnS, MnSe and the like acting as inhibitor are dispersed finely and uniformly. Therefore, when grain oriented electromagnetic steel sheet is produced by using such a thin cast sheet, the solution treatment finely precipitating MnS and MnSe before the hot rolling for obtaining good electromagnetic properties can be carried out at a lower temperature and hence various troubles generated when the temperature of the solution treatment is high can largely be mitigated and the operation is considerably improved. Since the solidification structure of the thin cast sheet is a granular structure having a uniform and small grain size, the electromagnetic steel sheet made from this thin cast sheet lacks the ridging resulted from the columnar crystal produced in the casting of molten metal and has less scattering of crystal orientation texture and improved electromagnetic properties. Moreover, the invention may be applied to the production of grain oriented electromagnetic steel sheets, but is advantageously applied to the production of non-oriented electromagnetic steel sheets because of the controlling of ridging and the improving of the electromagnetic properties. In general, when the Si content exceeds 5 wt%, the brittleness becomes conspicuous and the hot rolling and cold rolling are impossible. However, even when the thin cast sheet obtained according to the invention contains a great amount of Si as mentioned above, surface cracking is less and cold rolling is possible. In the invention, therefore, the Si content is within a range of 3.0-6.5 wt%, which is not a region at which working has taken place in the conventional technique, and the Mn content required for the precipitation of MnS and the like is not more than 2.5 wt%. 6 ○ Production of thin cast sheets of phosphor bronze alloy or high Sn copper alloy With phosphor bronze alloy or high Sn copper alloy, when the semi-solidified metal slurry is formed by strong agitation at a solid phase and liquid phase coexisting temperature region, the slurry has the primary granular solid particles suspended in the liquid phase. When such a semi-solidified metal slurry is cast in the twin roll type continuous strip caster, the cast structure of the thin cast sheet is a mixed structure consisting of primary solid particles existing as the solid phase in the slurry and fine granular crystal produced in the casting, so that there is formed no dendritic columnar crystal structure as observed in the continuous casting of molten metal. According to the invention, therefore, there can be obtained thin cast sheets having excellent surface properties without tin sweat and δ-phase. BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a graph showing the relation between penetration depth at which flows 80% of induction current and frequency in high frequency induction heating of a discharge nozzle; Fig. 2 is a microphotograph of a metal structure of a thin case sheet of SUS304 austenitic stainless steel containing B: 2.0% made from molten metal: Fig. 3 is a microphotograph of a metal structure of a thin case sheet of SUS304 austenitic stainless steel containing B: 2.0% made from semi-solidified metal slurry; Fig. 4 is a graph showing the relation of superheat of molten metal and solid fraction of semi-solidified metal slurry to the area ratio of columnar crystal occupying a section of thin cast sheet in SUS304 austenitic stainless steel containing B: 2.1%; Fig. 5 is a graph showing the relation of superheat of molten metal and solid fraction of semi-solidified metal slurry to area ratio of columnar crystal occupying a section of thin cast SUS430 ferritic stainless steel sheet; and Fig. 6 is a diagrammatic view illustrating an apparatus for the production of semi-solidified metal slurry by electromagnetic agitation, with discharge nozzle and a twin roll type continuous strip caster thereafter which is used in the examples. BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLE 1 An apparatus used in this example will be first described with reference to Fig. 6. Fig. 6 is a diagrammatic view illustrating apparatus for the production of semi-solidified metal slurry by electromagnetic agitation, a discharge nozzle and a twin roll type continuous strip caster. In this figure, numeral 1 is a tundish, numeral 2 a cooling agitation tank provided with a water-cooled jacket, numeral 3 an electromagnetic agitation coil disposed around the outer periphery of the cooling agitation tank, numeral 4 a core stopper, numeral 5 a discharge nozzle, numeral 6 a high frequency heating coil disposed around the outer periphery of the discharge nozzle 5, numeral 11 the respective rolls of a twin roll type continuous strip caster, numeral 12 a hydraulic cylinder for adjusting the distance between the two rolls, numeral 13 a lifting device for adjusting the positions of the two rolls in up and down directions, numeral 14 a pouring basin portion just above roll nip portion, numeral 21 molten metal, numeral 22 a semi-solidified metal slurry, and numeral 23 a thin cast sheet. The discharge nozzle 5 is made from alumina graphite, and the two rolls 11 are water-cooled copper rolls and have dimensions of roll diameter: 400 mm, roll width: 205 mm, roll distance: 0-30 mm and roll revolution number: 5-50 rpm. The production of the semi-solidified metal slurry and the production of the thin cast sheet by continuous casting are conducted by using this apparatus as follows. Molten metal is continuously fed from a vessel (not shown) to the tundish 1. The molten metal 21 fed into the tundish 1 flows downward in the cooling agitation tank 2, where it is agitated under an action of electromagnetic force of the electromagnetic agitation coil 3 (power: 700 KVA, magnetic flux density: 1000 gauss) while cooling to produce the semi-solidified metal slurry 22. The semi-solidified metal slurry 22 is fed into the pouring basin portion 14 of the twin roll type continuous strip caster through the discharge nozzle 5 induction-heated by means of the high frequency heating coil 6 (frequency: 100 kHz, power: 20 kW) while controlling the flow rate by adjusting up and down movements of the core stopper 4, to be cooled and solidified by the two rolls 11 to form the thin cast sheet 23. For austenitic stainless steels of SUS310S (Cr: 25 wt%, Ni: 21 wt%) and SUS316L (Cr: 17 wt%, Ni: 14 wt%, Mo: 2.5 wt%), semi-solidified metal slurrys are produced in the above apparatus by varying the solid fraction within a range of not more than 0.45. Then, thin cast sheets having a thickness: 3-12 mm are produced by the continuous casting from the above semi-solidified metal slurrys and molten metal used for the comparison, respectively, and then the castability (operability) and the structure and degree of surface cracking of the resulting cast sheet are evaluated. The evaluated results are shown in Table 1 together with the production conditions. In Samples Nos. 2, 3, 4, 6 and 7 (acceptable examples) of Table 1, the cast structure is a mixed structure of primary solid particles and fine granular crystal, and surface cracking of the thin cast sheet is not observed and the castability is good. On the contrary, with Sample No. 1 having a solid fraction of 0% (complete molten metal), the cast structure is comprised of coarse columnar crystal and surface cracking frequently occurs. Furthermore, in Sample No. 5 having a solid fraction of 0.45, the fluidity of the semi-solidified metal slurry is poor and the casting cannot be conducted, while in Sample No. 8 (thickness exceeds 10 mm), the cast structure is a mixed structure consisting of primary solid particles and coarse granular crystal, so that surface cracking is somewhat observed. Moreover, in the acceptable examples according to the invention, when final products are obtained by subjecting to cold rolling and annealing treatment according to the usual manner, the quality is confirmed to be substantially the same level as with a conventional product obtained by steel ingot - blooming - hot rolling. EXAMPLE 2 A semi-solidified metal slurry is produced by varying the solid fraction within a range not exceeding 0.45 in SUS304 austenitic stainless steel containing B: 0.5-5.0 wt% with the use of the apparatus used in Example 1. Then, thin cast sheets having a thickness: 5-12 mm are produced by continuous casting from the above semi-solidified metal slurries and molten metal used for comparison, respectively, and then the castability (operability) and columnar crystal occupying area ratio in a section and the presence or absence of surface cracking in the resulting cast sheet are evaluated. The evaluated results are shown in Table 2 together with the production conditions. Sample No. B content (wt%) Thickness of thin cast sheet (mm) Solid fraction Castability Columnar crystal occupying area ratio (%) Surface cracking of thin cast sheet Remarks 1 0.5 5 0.05 good 0 A Acceptable Example 2 2.0 10 0.10 good 0 A Acceptable Example 3 3.0 8 0.20 good 0 A Acceptable Example 4 4.0 8 0.30 good 0 A Acceptable Example 5 2.0 8 0.40 good 0 A Acceptable Example 6 2.0 8 0 (ΔT=60°C) good 80 C Comparative Example 7 2.0 8 0.45 not castable - - Comparative Example 8 2.0 12 0.20 good 50 B Comparative Example 9 5.0 8 0.30 good 0 C Comparative Example Moreover, surface cracking of the thin cast sheet in Table 2 is evaluated according to the following standard. A: no cracking B: slight cracking C: much cracking As seen from Table 2, the castability is poor, or the formation of columnar crystal or the occurrence of surface cracking in the thin cast sheet is observed in the comparative examples, while in the examples according to the invention, the castability is good and formation of columnar crystal or the occurrence of surface cracking is not observed in the resulting thin cast sheets. Moreover, when the B content is 5.0 wt% (Sample No. 9), surface cracking occurs in the thin cast sheet, while when it is 4.0 wt% (Sample No. 4), no cracking occurs, so that the upper limit of the content is preferably 4.0 wt%. Furthermore, when the thin cast sheets obtained by the method according to the invention (Samples Nos. 1-5) are annealed (1150°C·1 hour) - pickled and cold-rolled at a draft: 40-60% to produce final products, there can always be obtained cold rolled sheets having excellent surface properties without cracking. EXAMPLE 3 A semi-solidified metal slurry is produced by varying the solid fraction within a range of not more than 0.45 in SUS430 and SUS430LX ferritic stainless steels with the use of the apparatus used in Example 1. Then, thin cast sheets having a thickness: 4-15 mm are produced by continuous casting from the above semi-solidified metal slurrys and molten metal used for the comparison, respectively, and then the columnar crystal occupying area ratio in a section of the resulting cast sheet is evaluated. Furthermore, these thin cast sheets are subjected to an annealing at a temperature: 950°C and cold-rolled at a draft: 75-80%. Thereafter, these cold rolled sheets are annealed at a temperature: 750-850°C and then subjected to pickling. The thus obtained steel sheets are subjected to deep drawing into a cylinder of 100 mm diameter and the degree of ridging occurrence is measured by surface observation. The measured results are shown in Table 3 together with the production conditions. Moreover, the ridging in Table 3 is evaluated according to the following standard. A: no ridging B: slight ridging C: much ridging As seen from Table 3, the formation of columnar crystal and the occurrence of the ridging are observed in the comparative examples, while in the thin cast sheets obtained by the method according to the invention (acceptable examples), the columnar crystal occupying area ratio is 0% and there is observed no occurrence of the ridging on the surface of the deep drawn product after the working to the steel sheet. EXAMPLE 4 A semi-solidified metal slurry is produced by varying the solid fraction within a range of not more than 0.45 in SUS440C martensitic stainless steel (C: 1.1 mass%, Cr: 17.0 mass%) with the use of the apparatus used in Example 1. Then, thin cast sheets having a thickness: 3-12 mm are produced by continuous casting from the above semi-solidified metal slurries and molten metal used for the comparison, respectively, and then the castability (operability) and the structure and degree of surface cracking in the resulting cast sheet are evaluated. The evaluated results are shown in Table 4 together with the production conditions. Moreover, the coarse carbide is not formed in all of the above cases as the structure observations show. In Samples Nos. 2, 3, 4, 6 and 7 (acceptable examples) of Table 4, the cast structure is a mixed structure consisting of primary solid particles and fine granular crystal, and surface cracking of the thin cast sheet is not observed and the castability is good. On the contrary, in Sample No. 1 having a solid fraction of 0% (complete molten metal), the cast structure is comprised of coarse columnar crystal and surface cracking frequently occurs. Furthermore, in Sample No. 5 having a solid fraction of 0.45, the fluidity of the semi-solidified metal slurry is poor and the casting can not be conducted, while in Sample No. 8 (thickness exceeds 10 mm), the cast structure is a mixed structure consisting of primary solid particles and coarse granular crystal, so that surface cracking is somewhat observed. Moreover, in the acceptable examples according to the invention, when final products are obtained by subjecting to cold rolling and annealing treatment according to the usual manner, the quality is confirmed to be at substantially the same level as with the conventional product obtained through steel ingot - blooming - hot rolling. EXAMPLE 5 A semi-solidified metal slurry is produced by varying the solid fraction within a range of not more than 0.45 in silicon steel containing Si of 3.3 wt% or 6.5 wt% with the use of the apparatus used in Example 1. Thin cast sheets having a thickness: 3-12 mm are produced by the continuous casting from the above semi-solidified metal slurrys and molten metal used for the comparison, respectively, and then the castability (operability) and the structure and degree of surface cracking in the resulting cast sheet are evaluated. The evaluated results are shown in Table 5 together with the production conditions. In Sample Nos. 2, 3, 4, 6, 7 and 9 (acceptable examples) of Table 5, the cast structure is a mixed structure consisting of primary solid particles and fine granular crystal, and surface cracking of the thin cast sheet is not observed and the castability is good. On the contrary, in Samples Nos. 1 and 10 having a solid fraction of 0% (completely molten metal), the cast structure is comprised of coarse columnar crystal and surface cracking frequently occurs. Furthermore, in Sample No. 5 having a solid fraction of 0.45, the fluidity of the semi-solidified metal slurry is poor and casting cannot be conducted, while in Sample No. 8 (thickness exceeds 10 mm), the cast structure is a mixed structure consisting of primary solid particles and coarse granular crystal, so that some surface cracking is observed. When these thin cast sheets are cold-rolled at a draft: 40%, all thin cast sheets exhibiting surface cracking cause cracking during the cold rolling, while all thin cast sheets exhibiting no surface cracking do not cause cracking even during cold rolling to produce normal cold rolled products. EXAMPLE 6 A semi-solidified metal slurry is produced by varying the solid fraction within a range of not more than 0.45 in phosphor bronze alloy containing Sn: 3.5-9.0 wt% and P: 0.03-0.35 mass% or high Sn copper alloy containing Sn: 10-25 mass% with the use of the apparatus used in Example 1. Then, thin cast sheets having a thickness: 3-12 mm are produced by continuous casting from the above semi-solidified metal slurrys and molten metal used for the comparison, respectively, and then castability (operability), solidification structure of the resulting thin cast sheet, degree of tin sweat and the state of forming coarse δ-phase are evaluated. The evaluated results are shown in Table 6 together with the production conditions. As to the phosphor bronze alloys of Table 6, in Samples Nos. 2, 3, 5 and 8 (acceptable examples), the solidification structure is a mixed structure consisting of primary solid particles and fine granular crystal, and the occurrence of tin sweat and the formation of coarse δ-phase are not observed, and the castability is good. On the contrary, in Sample No. 1 having a solid fraction of 0% (complete molten metal), the solidification structure is comprised of coarse columnar crystal and tin sweat occurs and the formation of coarse δ-phase is observed. Furthermore, in Sample No. 4 having a solid fraction of 0.45, the fluidity of the semi-solidified metal slurry is poor and casting cannot be conducted, while in Samples Nos. 6, 7 (thickness exceeds 10 mm), the solidification structure is a mixed structure consisting of primary solid particles and coarse granular crystal (larger than the granular crystal in the acceptable examples), so that the occurrence of tin sweat cannot be controlled and some formation of coarse δ-phase is observed. Moreover, in the acceptable examples using phosphor bronze alloy according to the invention, when final products are obtained by subjecting to soaking treatment, cold rolling and the like according to the usual manner, the quality is confirmed to be substantially the same as in the case of being subjected to grinding over full surface. As to the high Sn copper alloy, in Sample Nos. 9 (Sn: 10%) and 10 (Sn: 14%) having a Sn content of not more than 14 wt%, the solidification structure is a mixed structure consisting of primary solid particles and fine granular crystal likewise the acceptable examples of the phosphor bronze alloy, and the occurrence of tin sweat and the formation of coarse δ-phase are not observed, and the castability is good. In Sample No. 11 wherein the Sn content is increased to 20 wt%, the coarse δ-phase is slightly observed, but the thin cast sheet may be rendered into a final product by soaking treatment and cold rolling. However, in Sample No. 12 wherein the Sn content is further increased to 25 wt%, a great amount of the coarse δ-phase is formed and cracking is frequently caused in the cold rolling and the final product cannot be obtained. Therefore, the Sn content is preferably not more than 20 wt%. INDUSTRIAL APPLICABILITY According to the invention, the production of thin cast sheets having an excellent quality by the continuous casting of semi-solidified metal slurry becomes easy. Furthermore, the following effects are obtained by producing thin cast sheets from various metal materials according to the invention, so that the invention is very useful in the production of sheet products made from the respective metal materials. 1 ○ Austenitic stainless steel Thin sheets of austenitic stainless steel having no surface cracking can be produced, and the product yield can largely be improved to considerably reduce the cost. 2 ○ Boron-containing austenitic stainless steel With boron-containing austenitic stainless steel not subjected to hot working to any significant extent, the production of thin cast sheets having a good workability is easy and hot working can be omitted, so that there is very great benefit in the production of thin sheets. 3 ○ Ferritic stainless steel The production of raw material for ferritic stainless steel thin sheets without ridging occurring in the forming work of the thin sheet becomes easy, and the yield in the forming work of the thin sheet is improved while the cost is largely reduced. 4 ○ Martensitic stainless steel Thin cast martensitic stainless steel sheet lacking formation of coarse carbide can easily be produced, so that the production of thin sheet products having a high quality and a low cost can be realized. 5 ○ Silicon steel Thin cast silicon steel sheet having a fine granular structure without segregation, a good internal quality with precipitates of MnS and the like finely dispersed, and less surface cracking and work cracking can be produced, so that one can expect a decrease in the temperature of solution treatment when producing grain oriented electromagnetic steel sheet and improvement in the electromagnetic properties of an electromagnetic steel sheet as well as ease of production of 6.5% Si thin steel sheet, which has been produced by complicated steps in the conventional technique. 6 ○ Phosphor bronze alloy and high Sn copper alloy Thin cast sheets of phosphor bronze alloy and high Sn copper alloy having good quality without tin sweat and work cracking can be produced. Furthermore, a grinding treatment is generally not required, so that improvement of product yield and the simplification of working steps can be attained, and the cost can largely be reduced.

Claims: A method of producing thin cast sheets by continuous casting in a continuous production device having an upper portion and a lower portion, which comprises continuously feeding molten metal into a said upper portion of the continuous production device, agitating said molten metal in said continuous production device, cooling said molten metal during said agitating to form in said continuous production device a semi-solidified metal slurry and feeding the semi-solidified metal slurry through a discharge nozzle arranged at the bottom portion of the continuous production device, said nozzle being heated, onto a twin roll type continuous strip caster at which the slurry is rapidly quenched and cast to form a thin cast metal sheet having a fine structure and a dispersed precipitate therein, characterised in that said cooling is carried out to yield a semi-solidified metal slurry having a solid fraction of 0.01 to 0.40 in which fine non-dendritic primary solid particles are suspended and in that said nozzle is heated through high frequency induction heating utilizing a frequency of 40-200 kHz so that not less than 80% of the supplied induction current is applied to said nozzle. A method of producing thin cast sheets by continuous casting as claimed in claim 1, wherein the agitation is effected by an electromagnetic agitating system. A method of producing thin cast sheets by continuous casting as claimed in claim 1, wherein the agitation is effected by an agitator rotating system. A method of producing thin cast sheets by continuous casting as claimed in any one of claims 1 to 3, wherein the discharge nozzle is made from alumina graphite having a specific resistance of 5000 µΩ·cm - 12000 µΩ·cm. A method of producing thin cast sheets by continuous casting as claimed in any one of claims 1-4, wherein the thin cast metal sheet has a thickness of not more than 10mm. A method of producing thin cast sheets by continuous casting as claimed in any one of claims 1-5, wherein said molten metal is an austenitic stainless steel containing P and S so that said thin metal sheet exhibits a fine dispersion of P and S. A method of producing thin cast sheets by continuous casting as claimed in any one of claims 1-5, wherein said molten metal is a B, P and S containing austenitic stainless steel containing 0.5-4.0 wt.% of B so that said thin metal sheet exhibits a fine dispersion of P and S in the form of boride. A method of producing thin cast sheets by continuous casting as claimed in any one of claims 1-5, wherein said molten metal is a ferritic stainless steel and the casting is carried out so that the formation of columnar crystal in the thin cast metal sheet is prevented. A method of producing thin cast sheets by continuous casting as claimed in any one of claims 1-5, wherein said molten metal is a martensitic stainless steel and the casting is carried out to obtain a fine dispersion of carbide. A method of producing thin cast sheets by continuous casting as claimed in any one of claims 1-5, wherein said molten metal is a silicon steel containing Si: 3.0-6.5 wt.% and Mn to not more than 2.5 wt.% and the casting is carried out so that the cast thin metal sheet exhibits a fine structure and a fine dispersion of a Mn-containing compound. A method of producing thin cast sheets by continuous casting as claimed in any one of claims 1-5, wherein said molten metal is a phosphor bronze alloy containing Sn: 3.5.-9.0 wt.% and P: 0.03-0.35 wt.% and the casting is carried out so that the formation of columnar crystal and fine structure in said thin cast metal sheet is prevented. A method of producing thin cast sheets by continuous casting as claimed in any one of claims 1-5, wherein said molten metal is a high Sn copper alloy containing Sn: 8-20 wt.% and the casting is carried out so that formation of columnar crystal and fine structure in said cast metal sheet is prevented. A method of producing thin cast metal sheets by continuous casting as claimed in any preceding claim, wherein said nozzle has a wall thickness of 15-40mm.

Inventors: HIRONAKA KAZUTOSHI, MORIYA TAKAHARU, MURAKI MINEO, MURATA YASUYUKI, NANBA AKIHIKO, NISHIIKE UJIHIRO, YOSHIDA CHISATO, YOSHIDA NAOTSUGU, HIRONAKA, KAZUTOSHI, MORIYA, TAKAHARU, MURAKI, MINEO, MURATA, YASUYUKI, NANBA, AKIHIKO, NISHIIKE, UJIHIRO, YOSHIDA, CHISATO, YOSHIDA, NAOTSUGU, HIRONAKA, KAZUTOSHI HIPPON YAKIN KOGYO CO., LTD., MURAKI, MINEO, KAWASAKI STEEL CORP., MURATA, YASUYUKI, TOPY INDUSTRIES LIMITED, NANBA, AKIHIKO RHEO-TECHNOLOGY LTD., NISHIIKE, UJIHIRO, KAWASAKI STEEL CORP., YOSHIDA, NAOTSUGU, SUMITOMO METAL INDUSTRIES LTD.
Applicants: RHEO TECHNOLOGY LTD, RHEO-TECHNOLOGY, LTD
Application year: 1994
Main classification: B22D11
IPC codes: B22D11